This invention concerns an electronic power supply for driving an electroacoustic transducer at its resonant ultrasonic frequency, typically a predetermined frequency in the range between 18 kHz and 60 kHz. More specifically, this invention refers to a power supply for driving a piezoelectric transducer provided with a horn at its parallel resonant frequency, such transducer and horn combination being used most frequently for welding thermoplastic parts, requiring power from several hundred watts to several kilowatts for time intervals ranging from several milliseconds to a few seconds. A typical prior art power supply of this type is shown in U.S. Pat. No. 3,432,691 issued to A. Shoh, dated Mar. 11, 1969, entitled "Oscillatory Circuit for Electroacoustic Converter". The power supply disclosed hereafter incorporates improvements and novel features not present in the prior art supply, such features being necessitated by the requirement that ultrasonic welding apparatus be operable by computer control at high speed and under conditions of high precision and repeatability involving varying workpieces and production runs.